1. Field of the Invention
The present invention relates generally to towed-booster orbital launch systems, and more particularly to an orbital launch technique that transfers kinetic energy from the tow aircraft to a towed launch aircraft and booster using a maneuver.
2. Description of the Related Art
Launch of a payload, such as a spacecraft or satellite, into orbit has traditionally been performed by accelerating the spacecraft from the ground to a required elevation using booster rockets that contain enough thrust to lift the spacecraft. Since the energy required to continue lifting the spacecraft and the booster rocket decreases as the fuel is expended and as air resistance decreases, the boosters are frequently staged and have a fuel supply much larger for earlier stages than for subsequent stages. Further, the vertical launch structure required for such launches is costly and constrains the initial location of the launch to particular launch sites. FIGS. 1A-1B illustrate such a prior art vertical ground-based rocket launch, with a launch tower 2 supporting booster rocket 4 (and attached payload), until the ignition of booster rocket 4 as shown in FIG. 1B.
In order to remove launch constraints and to attempt to reduce the size and cost of booster rockets used in ground-based launches, aircraft have been used to carry the booster rocket and attached payload to elevations near typical aircraft ceilings before the booster rocket is ignited. FIGS. 2A-2C illustrate such a prior art launch technique, in which booster rocket 4 is released from the underside of a launch aircraft 6. Booster rocket 4 is ignited after release, and gains the benefit of added launch elevation, e.g., 30,000-40,000 feet, and an initial horizontal velocity supplied by the launch aircraft, i.e., 600 mph. Typically, the gains of air launch are greatly reduced over theoretical gains, because the booster rocket must be reinforced to provide for horizontal carrying of the booster rocket. The booster rocket also typically requires a wing in order to make the turn needed to increase the pitch angle of booster rocket 4 as shown in the transition to that of FIG. 2C from that of FIG. 2B, which aligns the flight path of booster rocket 10 to the required orbit.
More recently, techniques have been developed using a tow aircraft to tow a towed launch glider. Such techniques improve launch efficiency, i.e., the ratio of the mass of the payload that reaches orbit to the total pre-launch mass of the booster rocket with payload, by carrying the booster rocket to a greater launch elevation. Such techniques provide improvement because the lift provided by the glider is greater than the lift that can be provided by launch aircraft 6 in FIGS. 2A-2C. Such a technique is illustrated in FIGS. 3A-3C, in which aircraft 6 is used to tow a towed launch aircraft 8, under which booster rocket 4 is mounted. A tow line 9 couples aircraft 6 to towed launch aircraft 8 until the desired launch elevation is reached, at which time tow line 9 is released. Then, towed launch aircraft 8 pulls up slightly to orient booster rocket 4 above horizontal, as shown in FIG. 3B. Finally, booster rocket 4 is dropped and ignited as shown in FIG. 3C.
However, even with all of the improvements that have been made to date, launch of a payload into orbit is an extremely costly operation and an improvement in launch efficiency can significantly reduce the fuel and booster cost. Therefore, it would be desirable to provide a further improved towed launch of a booster and payload.